REPORT ON THE HEXACTINELLIDA. 191 



imperforate limiting zones lie just above the gastral septa, and arc indeed partially- 

 united with them. This union of the septa of the gastral cavity with the sieve-plate 

 usually occurs only in the centre and at the outer ends, so that one can see 

 the septa, in their middle portion, ending with a free convex margin just below 

 the plate. In some cases, however, each septum unites along its whole extent 

 with the corresponding zone of the sieve-plate. Commensal Anthozoa occur in 

 extremely variable number all over the lateral surface, and even on the cruciate 

 imperforate areas of the sieve-plate. They protrude from circular apertures, which are 

 2 to .3 mm. in width and possess a firm peripheral layer. In some specimens they^ 

 cover the whole lateral surface so thickly that the distance between them is not more 

 than from 3 to 10 mm., while they also occur aljundantly (PI. XXVII. fig. 2) on the 

 dividing zones of the sieve-plate. In other specimens they occur singly only here and 

 there, though they never seem to be wholly absent. 



Apart from these Anthozoa tubes, the surface of the s^ionge is comparatively smooth. 

 The 23ores of the dermal membrane covering the surface are for the most part micro- 

 scopically small. The extreme inferior end of the body, adjacent to the Palythoa 

 encrustating the basal tuft, consists of an inconspicuous but compact circular cushion, 

 varying greatly in breadth in the difi"erent specimens. In dried forms, the fine reticulate 

 pattern which is distinctly seen over the whole lateral surface of the body is in this 

 region no longer recognisable. 



When the terminal sieve-plate is removed, the gastral cavity is revealed, and is 

 seen to be di\'ided into four wide spaces by the four broad, cruciately disposed, radial 

 septa, with a central conical elevation. The free upper margins of these septa, if not 

 fused with the sieve-plate, are rounded off and convex, and extend from the body 

 margin on the outside to the conical elevation in the centre. The cavities lying 

 between the septa become gradually narrower downwards, and receive from the 

 sides and from below the wide lacunar efferent canals of the body-wall. In a longi- 

 tudinal section near the central axis, a continuation of the basal tuft is seen as a some- 

 what markedly twisted strand of spicules, prolonged up the central column and gradually 

 narrowing towards the conical elevation, as was indeed long ago observed and figured by 

 Max Schultze. Since the efferent canals are very wide and often lacunar, and since the 

 subdermal spaces with the afferent canals penetrating inwards from them have a similar 

 character, it evidently follows that the layer betw^een the two systems of canals cannot 

 be by any means broad. 



The skeletal elements of Hijalonema sieholdii which always remain completely 

 isolated were thoroughly studied in 1860 by Max Schultze, and so excellently described 

 in his monograph, that I must here simply content myself wdth referring to that admirable 

 memoir, and with a brief review of the different forms of spicules and of the manner of 

 their disposition. 



